Motor drives and other power conversion systems typically receive AC input power from a grid or other power source and convert that to DC for driving a load such as a DC motor, or an inverter may convert the DC to AC output power for driving an AC motor or other load. Many such power converters include active front end (AFE) rectifier stages with switching devices to rectify the input AC power. In many power conversion control schemes, synchronization of the front end converter operation with the phase angle of the input AC power is desirable, for instance, in performing power factor control, attempting to minimize total harmonic distortion, etc. Synchronizing power converters to the grid power is typically done using phase locked loops (PLLs) to provide a phase signal to the converter controller based on sensed phase voltages. However, many phase voltage sensors suffer from low frequency or DC offsets or drift, as well as higher order harmonics, and imbalance in the voltage which can corrupt the phase signal output from the phase lock loop.
Some solutions to this problem employ one or more tracking filters in the feedback loop of a PLL to eliminate error signal disturbances. Examples are illustrated and described in U.S. patent application Ser. No. 12/627,472 to Kerkman et al., filed Nov. 30, 2009, published on Jun. 2, 2011 as Publication No. US 2011/0128054 A1 entitled “PHASE LOCK LOOP WITH TRACKING FILTER FOR SYNCHRONIZING AN ELECTRIC GRID”, and assigned to the assignee of the present disclosure, which is hereby incorporated by reference in its entirety. However, difficulties may arise in implementing harmonic tracking filters in conjunction with a tracking filter at the fundamental frequency due to filter bandwidth overlap or proximity, and this approach may thus yield bandwidth limitations for the PLL.
U.S. patent application Ser. No. 12/706,488 to Kerkman et al., filed Feb. 16, 2010, published on Aug. 18, 2011 as Publication No. US 2011/0199072 A1 entitled “POWER CONTROL SYSTEM AND METHOD”, and assigned to the assignee of the present disclosure provides electrical device control systems and methods using tracking filter devices, and U.S. patent application Ser. No. 12/627,400 to Seibel et al., filed Nov. 30, 2009, published on Jun. 2, 2011 as Publication No. US 2011/0130993 A1 entitled “DIGITAL IMPLEMENTATION OF A TRACKING FILTER”, and assigned to the assignee of the present disclosure presents a digital implementation of such a tracking filter. The entireties of these published applications are hereby incorporated by reference. Other solutions to the above shortcomings may include low pass filter nulling of the DC offset, but this requires additional hardware thereby increasing the cost, size and weight of power converter systems.
Accordingly, there is a continuing need for improved techniques and apparatus for controlling offsets in grid synchronization PLLs and in other applications in which offsets need to be removed from an input signal.